Disk system and power-on sequence for the same

ABSTRACT

With respect to the disk drives provided power from a single power source in a disk drive system, start up power is first supplied to a first start-up group of the disks, preferably comprising all of the master disks, with the size of the group being selected so that the required current does not exceed the capacity of the power source. When the disk drives of the first group have substantially reached steady state, start-up is conducted with respect to a second start-up group of the disk drives so that the current required during start-up for the second group and the current required for steady state drive of the first start-up group does not exceed the capacity of the power source. With respect to each start-up group, the number of disk drives is the maximum integer value and decreases or remains the same with respect to subsequent start-up groups. When simultaneously transferring subdivided data in parallel to all of the disk drives of a parity group, respectively, seek operations in such a system are prevented from occurring simultaneously by offsetting the indices on the disks, by varying the seek operation start timing or by varying the head addresses for the start of reading and writing, all within one revolution, but the seek operations are ended at the same time, to reduce peak current requirements of the power source.

This is a continuation application of U.S. Ser. No. 10/122,301, filedApr. 16, 2002 now U.S. Pat. No. 6,625,690; which is a continuationapplication of U.S. Ser. No. 09/788,392, filed Feb. 21, 2001, now U.S.Pat. No. 6,397,294; which is a continuation application of U.S. Ser. No.09/522,015, filed Mar. 9, 2000, now U.S. Pat. No. 6,286,108; which is acontinuation application of U.S. Ser. No. 09/357,372, filed Jul. 20,1999, now U.S. Pat. No. 6,131,142; which is a continuation applicationof U.S. Ser. No. 08/934,201, filed Sep. 19, 1997, now U.S. Pat. No.6,012,124; which is a continuation application of U.S. Ser. No.08/472,460, filed Jun. 7, 1995, now U.S. Pat. No. 5,673,412; which is adivisional application of U.S. Ser. No. 07/725,672, filed Jul. 3, 1991now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an external memory unit for a computer orhigh-performance computer system, and more particularly to an array disksystem employing a large number of small disk drives and a maximum powersupply current requirement control, e.g. with respect to power on orhead seek.

In current computer systems, the data required by the host side, e.g.,by the CPU (central processing unit), is stored in a secondary storagesystem and the data is written to and read from the secondary storagesystem as required by the CPU.

The increasing sophistication of information systems in recent years hasled to a need for higher performance secondary storage systems. Oneanswer to this need is the array disk system which, as will be clearfrom the following description, consists of a large number of relativelysmall capacity magnetic disk drives. The array disk system is used forconducting parallel processing. Specifically, the data transferred fromthe CPU is subdivided and simultaneously stored in a plurality ofmagnetic disk drives and, during data read, the subdivided data issimultaneously read from the magnetic disk drives regenerated to obtainthe original data from the data read simultaneously from the disk drivesand transferred to the CPU at high speed. The magnetic disk drives thatcarry out this parallel processing are divided into groups as indicatedin FIG. 12( a). Each group constitutes a unit within which all membermagnetic disk drives operate in the same manner.

The secondary storage system generally uses nonvolatile storage media,typically magnetic disk drives, optical disk drives or the like.

This type of array disk system is discussed, for example, by D.Patterson, G. Gibson and R. H. Kartz in a paper titled A Case forRedundant Arrays of Inexpensive Disks (RAID) read at the ACM SIGMODConference, Chicago, Ill., (June 1988). This paper reports on theresults of studies into the performance and reliability of both arraydisk systems which subdivide and process data parallely and array disksystems which independently treat distributed data. The two array disksystems referred to in this paper are considered to be the most commontypes in use today.

The array disk system which subdivides data and processes the subdivideddata parallely will now be explained. The array disk system has a largenumber of relative small capacity magnetic disk drives. As shown in FIG.14, the data transferred from the CPU is subdivided and simultaneouslystored in parallel in a plurality of data disk drives 7 and a paritydisk drive 8 that constitute a parity group 4. During data read, theprocedure is reversed, i.e., the subdivided data is simultaneously readin parallel from the disk drives regenerated to obtain the original datafrom the data read simultaneously from the disk drives and transferredto the CPU. This parallel processing enables the data to be transferredat high speed. For enhancing the reliability of the array disk system,parity data is generated from the subdivided data and stored in theparity disk drive P (8). In this way, when a problem arises making itimpossible to read data from one of the magnetic data disk drives D (7)among those in which the subdivided data is stored, the data stored inthe disabled magnetic disk drive can be reconstructed from the datastored in the remaining magnetic disk drives 7 and the parity data ofdisk drive 8. The provision of parity disks is necessary for improvingthe reliability of a system which, like the array disk system, consistsof a large number of magnetic disk drives.

Systems in which a high transfer rate is realized by simultaneouslyconducting reading and writing with respect to any array of disks aredisclosed in Japanese Unexamined Patent Public Disclosure 1(1989)-250158and Electronic Design, Nov. 12, 1987, p. 45. As shown in FIG. 2, thesetypes of systems define a plurality of disk drives 211–215 as an array.Preferably a rotation synchronize circuit 220 rotation-synchronizesthese disk drives with respect to an external reference clock or withrespect to one disk drive among the plurality of disks making up thearray. A sequencer 240 subdivides the data transferred from the host 210through an interface 230 into bits, bytes, blocks or some otherarbitrary unit, and also generates parity or other such ECC (errorchecking and correction) data. These data are written to the disk drives211–214 substantially simultaneously by disk drive control circuits 250.During regeneration, the sequencer 240 reconstructs the original datafrom data read simultaneously from the disk drives and outputs theregenerated data to the host through the interface 230. The buffer 260is situated between the control circuits 250 and the sequencer 240 forabsorbing rotational discrepancies among the disks. The interface 230,sequencer 240, control circuits 250 and buffers 260 are controlled by aprocessor 270.

When reading and writing of data are conducted with respect to N+1 disks(+1 indicating the parity disk 215) in this manner, the apparenttransfer rate becomes N times the transfer rate of the individual diskdrives. Moreover, the provision of a redundant disk (the parity disk 215in this example) makes it possible to ensure accurate data regenerationeven if one disk drive should break down.

Further, as shown in FIG. 3, COMPCON '89 Spring, February 1989, p118discloses an arrangement in which a plurality of interconnected diskdrive arrays 281–284 (which will be referred to as parity groups) areeach constituted in the manner of FIG. 2. High-speed transfer isrealized by having the disks within the parity groups 281–284simultaneously conduct read and write operations. When a disk within agroup breaks down, the data is reconstructed within the group concerned.This reference further discloses the formation of separate groups291–295 (which will be referred to as power groups) constitutedperpendicular to the parity groups. Each power group constitutes aseparate unit as regards the supply of electric power for the diskdrives and the cooling fans. This arrangement limits the effect of thebreakdown of a single power group to making it impossible to read thedata of only one disk in each parity group. As a result, the aforesaiddata error checking and restoration capability remains intact and thedata can be regenerated.

SUMMARY

The aforesaid arrangements do not, however, take into account the factthat the initial current becomes large when the large number of diskdrives are simultaneously started up. As shown in FIG. 4, the powersupply current required immediately after start-up of a disk drive ismore than twice that during steady state operation. This large currentfollowing start-up continues to flow for no more than several tens ofseconds. Assume that a single power supply serves D number of diskdrives (D being equal to the number of parity groups), that the steadystate current value is I(A), and that a current equal to k times thesteady state current is required immediately after start-up. The powersupply is thus required to be capable of supplying, albeit for only ashort period, a current of I×k×D (A).

Japanese Unexamined Patent Publication Disclosure 57(1982)-3265discloses a technique for staggering the times at which power-on isconducted with respect to the disk drives. While this method makes itpossible to reduce the required capacity of the power supply, itconsiderably prolongs the time required for start-up of the entiresystem when applied to a system which, like the array disk system, has alarge number of disk drives that have to be supplied with power.

An object of this invention is to provide an array disk system andcontrol the same to reduce the amount of electric current required bythe array disk system, e.g. the amount of electric current requiredthereby during a power-on sequence for the disk system which enables thedisk system to be started up within a prescribed period of time usingrelatively small power supplies.

For achieving this object, the present invention divides the disk driveswithin the disk system into a number of groups and separately starts upthe respective disk drive groups.

The number of disk drives constituting the individual groups ordinarilydecreases in the order that the groups are started up. This is because,for example, the reserve power of the power supply after the start-up ofthe first group is equal to the rated capacity of the power supply minusthe amount of current required for maintaining the disk drives of thefirst group in the steady state. It suffices to set the number of diskdrives in the first group to be started up so as not to exceed thecapacity of the power supply being used. This number can be decided bythe following method.

Assume that D disk drives are started up using a single power supply,that the steady current per disk drive in the steady state is I(A), andthat an initial current k times as large as the steady state current isrequired at the time of start-up. Then, if the number of disk drivesfirst started up is set at D/k, the current at the time of start-up willnot exceed the amount of current when all of the disk drives areoperating in the steady state, namely, will not exceed ID(A).

Next, the manner for determining the number of disks to be included inthe second and following groups to be started up will be explained.Basically, it suffices if the number of disk drives in the second andfollowing groups to be started up is such that the amount of currentrequired for starting up the disk drives does not exceed the reservecapacity of the power supply. For optimum effect, however, the followingmethod can be considered. After the first group of D/k disk drives havereached the steady state (e.g., after several tens of seconds), the nextgroup of disk drives is started up. It then basically suffices to setthe number x of disk drives in this next or second group as the numberobtained by dividing the reserve current capacity of the power supplywhen D/k disk drives are operating in steady state by kI. This can beexpressed by the following equation:x=1/k(1−1/k)D

Since only an integral number of disk drives is possible, any decimalamount in the value of D/k is dropped, i.e., the value obtained from theforegoing equation is rounded down. When this method is used fordetermining the numbers of disk drives, it may happen that a single diskdrive remains at the end. For starting up this disk drive, however, amaximum power supply current of I(D−1+k) (A) is sufficient.

One disk drive of a parity group is sometimes designated as a masterdisk and subjected to rotation synchronization. In such case, thismaster disk has to be started up prior to the other disks. If the numberof master disk drives is such that they can all be started upsimultaneously, therefore, the master disk drives are included in thefirst group to be started up. Alternatively, it is possible to start upthe master disk drives one by one before starting up the other disks.

Since the disk drive groups are started up at different times to preventoverlap of the initial currents, the maximum current output of the powersupply can be reduced. Since the disk drives are organized into a numberof groups, the disk system can be started up within a prescribed periodof time.

An example magnetic disk drive of a type illustrated herein requires amaximum current of 4.5 A, which breaks down to 1 A for rotating thedisks, 2.8 A for seek operation and 0.7 A for other purposes. When seekoperation occurs simultaneously with parallel processing in an arraydisk system consisting of a large number of such disk drives, a verylarge current becomes necessary. Moreover, as protection against poweroutages or other such mishaps that might occur during the operation ofsuch an array disk system, it is necessary to provide battery backup forenabling data in the course of storage to be completely stored. Forsupply of such a large amount of current, it is necessary to use a verylarge battery.

An object of this invention is to provide an array disk system andcontrol the same to reduce the amount of electric current required bythe array disk system, particularly the amount of electric currentrequired thereby during seek operation, and also in this way to reducethe capacity required of a battery provided as a backup power source foruse during power outages and the like.

For achieving the aforesaid object, the present invention provides anarray disk system, as shown, for example, in FIGS. 12( a), (b), and (c)that has a large number of disk drives divided into a plurality ofgroups provided with control such that the timing of the start of seekoperations for moving the read/write heads to change the track positionsat which the read/write heads are located is varied among at least someof the groups and such that, within each group, the timing of the startof seek operations is the same for all of the disk drives or is variedamong at least some of the disk drives.

The control for causing the seek operation start timing to vary amongthe groups or among the disk drives of a group can be provided byrotation-synchronizing the disk drives such that the positions ofindices provided on the disks as references for the start of dataread/write are offset among the groups or among the disk drives.

In this case, parallel processing can be readily conducted by providingthe controller with data processing which simultaneously stores thesubdivided data simultaneously transferred to the respective groups inbuffer memories within the respective groups and conducts read/writeprocessing of the data from the buffers in accordance with thepositional offset of the indices.

Alternatively, the control for causing the timing of the start of theseek operations to vary among the groups or among the disk drives of agroup can be provided, as shown in FIGS. 18( a) and (b) for example, bydeliberately offsetting the seek operation start timing among the groupsor among the disk drives, without offsetting the positions of theindices on the disks. Since all of the indices are positionally alignedin this case, there is the advantage that rotation synchronizationcontrol is easy to conduct.

Further, the control for causing the timing of the start of the seekoperations to differ among the groups or among the disk drives of agroup can be provided, as shown in FIGS. 19( a) and (b) for example, byvarying the head addresses for the start of data reading and writingamong the groups or among the disk drives, without offsetting thepositions of the indices on the disks among the groups. This methodsimplifies the control since the head addresses can easily be variedamong the groups by software techniques.

The control used by the invention for achieving the aforesaid objects isfurther characterized in that the seek operations for moving theread/write heads to change the track positions at which the heads arepositioned are prevented from occurring simultaneously in at least someof the disk drives.

For preventing seek operations from occurring simultaneously the controlwill offset the position of the indices on the disks, to vary the seekoperation start timing or vary the head addresses for the start ofreading and writing.

In preventing seek operations from occurring simultaneously, it ispreferable from the point of reducing electric power consumption todivide the large number of disk drives into group units, each of aplurality of the disk drives, to prevent seek operation from occurringsimultaneously among the groups, and to make the division of the diskdrives into groups such that the seek operations occur in differentgroups at different times within the period of one disk revolution andall of the seek operations occurring at different times are completedwithin the same period.

In a disk system which conducts parallel processing, the positionalrelationship among the heads situated over the disks is generally suchthat the many disk drives making up the system operate as if they werean integrated unit. Specifically, the disks are rotation-synchronizedwith each other and the heads operate such that their track positionrelationships are all the same. In such a system, if the many diskdrives which conduct parallel processing are divided into a number ofgroups and each group is treated as a separate read/write unit, the timefor conducting seek operation is offset among the groups so that theoccurrence of a large seek current by the simultaneous occurrence of themany seek currents in the individual disks can be avoided. Therefore,the supply of current to the array disk system as a whole is lowered andthe capacity required of a battery for providing backup power duringpower outages and the like can be reduced.

Offsetting the positions of the indices on the disks among the groupsmakes it possible to offset among the groups the timing at which seekoperation starts for data exchange between the heads and the tracksduring one revolution and, thereby, to hold the seek current to a lowlevel.

As explained above, a prescribed seek time is required within eachrevolution for conducting a seek operation. During this time, the diskcontinues to rotate irrespective of whether or not data is beingexchanged. It is thus preferable to make effective use of this periodduring which data is not being processed for carrying out the seekoperation separately in each group. If this expedient is adopted, then,by deliberately offsetting the timing at which the seek operation isconducted among the groups within this period, it becomes possible,without offsetting the positions of the indices, to use this period togood advantage and thus to reduce the seek current.

Since in one and the same disk drive the seek operation is conductedafter the head at a specific head address (e.g., the bottommost head inFIG. 2) has completed data exchange with a track on the disk, changingthe head address at which data read/write is started among the groupschanges the timing at which seek operation is conducted among thegroups, so that the seek current can be reduced.

Up to this point, the explanation has been directed to the case wherethe seek operation timing is varied among the groups. It is, however,similarly possible to reduce the seek current by varying the seekoperation timing among disk drives in one and the same group accordingto the above teachings.

Since reducing the seek current reduces the amount of electric powerthat has to be supplied to the array disk system as a whole, itdecreases the capacity required of the backup battery for providingpower during power outages and the like, increases the reliability ofsystem operation during such emergencies, and enables the equipment forsupplying power to be made more compact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view for explaining a disk system according to theinvention;

FIGS. 2 and 3 are schematic views useful in explaining problems solvedby the present invention;

FIG. 4 is a graph for explaining current characteristics of the spindlemotor of a disk drive immediately after start-up;

FIG. 5 is a block diagram for explaining a motor drive control circuit;

FIG. 6 is a graph for explaining variation of the power supply current;

FIG. 7 is a schematic view of a disk system for explaining theinvention;

FIG. 8 is a graph for explaining variation of the power supply currentin the invention;

FIGS. 9 and 10 are schematic views of systems for explaining theinvention;

FIG. 11 is a schematic view of another embodiment of the invention;

FIG. 12( a) is a block diagram of a system portion of the inventionrelating to data storage;

FIGS. 12( b) and (c) are diagrams for explaining data storage in thesystem of FIG. 12( a);

FIG. 13 is a schematic view of the interior of a data or parity diskdive;

FIG. 14 is a diagram for explaining parallel processing of data;

FIG. 15 is a timing chart relating to data storage in a data or paritydisk;

FIG. 16 is a block diagram showing the internal structure of a groupcontroller;

FIGS. 17( a) and 17(b) are diagrams for explaining another example ofdata storage in the present invention;

FIGS. 18( a) and 18(b) are diagrams for explaining another example ofdata storage in the present invention;

FIGS. 19( a) and 19(b) are diagrams for explaining another example ofdata storage in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 13 shows the internal structure of a data disk drive preferablyused throughout this disclosure, for example a data disk drive 7 or aparity disk drive 8. A number of disks 12 rotate about a common shaft17, and R/W (read/write) heads 13 for reading and writing from and tothe disks 12 are carried by an actuator 11. As used herein, the termhead will refer to a single head for one surface as well as a pair ofheads that service opposed surfaces of adjacent disks. The heads 13 arenumbered, from top to bottom, #1 to #8. At least one R/W head 13 isrequired per side for each disk 12. All of the R/W heads 13 are moved inunison by the actuator 11. When read or write is conducted, the CPU 1issues a data storage address and the R/W heads 13 go to this address.Specifically, a head selector 14 selects the head number correspondingto the head address included in the address issued by the CPU 1 and theactuator 11 carries out a seek operation by which the R/W head 13 ismoved to the track corresponding to the cylinder address. When access bythe storage address for the data has been completed, a path selector 16selects the path to the host and the data is read or written by R/Wcircuit 15.

In a conventional manner, the disk drives 7, 8, 211–215, 411–414, 440,551–560, shown in FIG. 13, are referred to with a head address 30, andaccess is with respect to a specific track 31 that defines a cylinderfor all of the disks 12. An index 32 is provided on one or more or allof the disks 12 of the disk drive positioned to the head address 30, andthere is a cylinder address 33. Each single head or head pair issupported on an arm 34 having its other end supported on the actuator11, which moves radially with respect to the shaft 17 that rotatablysupports the disks 12. As shown by the headed arrows, data, address andcontrol signal lines are connected to a suitable bus, for exampleleading to the host.

In the case of an array disk system having the array disk controllerADC2 and array disk unit ADU3 of FIG. 12( a) which conducts parallelprocessing, the CPU 1 issues the read or write request to a disk driveparity group 4 made up of a number of data disk drives 7 and a paritydisk drive 8 and which disk drive parity group 4 constitutes a singleparallel processing unit. Within the group 4, a read or write request isissued by the group controller GC5 to each data disk drive 7 and theparity disk drive 8, and read or write processing of the type describedabove is conducted simultaneously with respect to all of the data diskdrives 7 and the parity disk drive 8 in the group 4. For this purpose,it is necessary to rotation-synchronize the disks 12 of the data diskdrives 7 and parity disk drive 8 within the parity group 4 so that thesame address is always being accessed in the data disk drives 7 andparity disk drive 8 within one access period, thereby to control themulti-disk system 2, 3 exactly as if it were a single disk drive.

An explanation will now be given on the problems that arise duringreading and writing of data in this type of array disk system when thevolume of data to be handled at one time becomes great.

When data is stored in the data disk drives 7 and the parity disk drive8, it is first stored at the cylinder track under head #1 and thensuccessively stored at the same cylinder tracks under heads #2, 3, 4, 5,6, 7 and 8. When storage has been completed up to the cylinder trackunder head #8, the actuator 11 conducts a seek operation to move the R/Wheads 13 to the adjacent cylinder, wherein data is similarly storedsuccessively or in parallel at the tracks under heads #0, 1, 2, 3, 4, 5,6, 7, 8. Reading of the stored data is carried out in a similar manner.

Thus, in the array disk system, when the amount of data to besimultaneously processed in data disk drives 7 is larger than thecapacity of one cylinder or available area of the cylinder firstaccessed, an intermediate seek operation is necessary for moving to theadjacent track.

A system for the invention is shown in FIG. 1. The power supply current(in lines 9 from power supply 10 of FIG. 12( a) e.g.) per magnetic diskdrive immediately after start-up in this embodiment exhibits thecharacteristics shown in FIG. 4. The required power supply current, forthis example, is 2A during steady state operation and 4A during initialstate operation (start-up). The initial state current of 4A continues toflow for 30 seconds. The manner in which the magnetic disk drives arearrayed in this example of FIG. 1 is similar to that in the exampleillustrated in FIG. 3. Each parity group 310 includes five disk drives,each steady state power group 320 includes eight disk drives, and thetotal number D of disk drives is forty. As there are five power groups,without the application of this invention it would ordinarily benecessary to provide 5 power supplies, each with the capacity to supplyup to 4(A)×8 (drives)=32 (A). Being common to the whole system, theinterface 230 and the sequencer 240 should preferably be provided with apower supply separate from that for the magnetic disk drives so as toestablish a dual power system. Even though the buffer and controlcircuits are paired, with each pair connected with the disk drives of arespective power group, they are logical circuits and should thereforehave a different voltage power supply. If to the contrary they are to besupplied with decreased voltage from the power supply for starting upthe disk drives, it is necessary to take the current they require intoaccount in determining the required current capacity of the powersupply. Herein, when we talk of current capacity or the like of a powersupply we are really talking about that available for the disk drives.Although the current required by these logical circuits varies withtheir size, it is at any rate much smaller than the currents requiredfor driving the disk drive spindle motors. In this embodiment, thecurrent required for the logical circuits is not more than 0.3 A.

When the present invention is applied, the number and sequence of thedisk drives simultaneously started up in each power group 320 can be 4,2, 1, and 1. As shown in FIG. 1, the disk drives are organized from thetop down into groups 330, 340, 350 and 360, consisting of 4, 2, 1 and 1parity groups 310 of disk drive(s), respectively. Further, as can beseen in the motor drive control circuit shown in FIG. 5, the timebetween power switch-on of the overall system and the start of drivingof the disk spindle motors is set independently for each of the groups330, 340, 350 and 360 to prevent overlap of the initial currents amongthe groups. In the present embodiment, a delay of 30 seconds isestablished between successive groups. The spindle motors 380 of group330 are turned on by a driver 390 almost simultaneously with receipt ofthe power-on signal 370. The spindle motors 380 of group 340 are turnedon after a timer circuit 400 has counted off 30 seconds followingreceipt of the power-on signal 370. In the same manner, the spindlemotors 380 of group 350 are turned on after a delay of 60 seconds andthose of group 360 after a delay of 90 seconds.

The time course variation in the initial current of the power suppliesof the respective power groups during this process was as shown in FIG.6. As can be seen, it rose no higher than 18 A, which is roughly 50%less than that should the invention not have been applied. The timerequired for all of the disk drives to reach their rated rotationalspeed was 30 seconds×4 groups=2 minutes. After start-up the steady statecurrent was 16A. In comparison, if an attempt should be made to limitthe power supply current to the same level without application of theinvention, the disk drives would have to be started up one by one andthe time required for all of the disk drives to reach their ratedrotational speed would be 30 seconds×8 disk drives=4 minutes. Thereduction in time is thus also 50%.

In the foregoing explanation the numbers of disks simultaneously startedup were 4, 2, 1 and 1. If the power supplies have adequate capacity,however, it is alternatively possible to start up the disk drives inthree power groups of 4, 2 and 2 disks. While this increases therequired amount of power supply current to 20 A, it reduces the timerequired for all disk drives to reach their rated rotational speed to 1minute 30 seconds. From this it will be understood that the numbers ofdisks to be simultaneously started up can be varied in light of the sizeof the power supply and the required start-up time.

In FIG. 7, each disk drive is represented by a circle, and one diskdrive in each parity group is designated as a master and subjected torotation synchronization. Although the master can be any disk drive inthe parity group, it has to be brought up to the rated rotational speedahead of the other disk drives in the group. The method used when theinvention is applied in such a case will now be explained. There are 4parity groups (each parity group is in a single horizontal line) and 4power groups (each power group is in a vertical line). The disk drivesare of the same type as those in the first embodiment. In each paritygroup, the disk drive designated as the master is preferably started upbefore the others. In FIG. 7, the disk drives 411, 412, 413 and 414lying on the diagonal at the intersections between the respective paritygroups and power groups are selected as the masters. All of the mastersare simultaneously started up. Following this, the remaining disk drivesof group 420 are started up and thereafter the remaining disk drives ofgroup 430 indicated in the same figure (remaining means other than thosedisk drives already started up) are started up. The time variation inthe power supply currents of the respective power groups in this case isshown in FIG. 8, from which it will be noted that a maximum current of10 A suffices and the steady state current after start-up is 8 A. Ifstart-up should be carried out without application of the invention, 16A would be necessary. The invention thus produces a pronounced effect inreducing power supply requirements.

In contrast to the above examples, FIGS. 9 and 10 respectively relate tocases in which the number of parity groups is smaller and larger thanthe number of power groups. When a master disk drive 440 for rotationalsynchronization is designated in each parity group, the arrangement ofFIG. 9 results in some power groups including no master disk drive 440and that of FIG. 10 results in some power groups including more than onemaster disk drive 440. When the invention is applied to thesearrangements, it suffices to establish the start-up groups 450, 460 and470 shown in these figures and after start up of the masters, to startupthese groups 450, 460 and 470 in succession at a time interval equal tothe time required for the disk drives to reach the prescribed rotationalspeed following start-up. It can be easily understood that the effect ofthe invention is obtainable with this arrangement. Therefore, thestart-up sequence is: the master disk drives 440 are all first startedup during a first period of time; and this is followed by a secondperiod of time wherein the disk drives other than the master disk drives440 are started up within start up group 450; thereafter, the diskdrives other than the master disk drives 440 are then started up instart up group 460; and thereafter the disk drives other than the masterdisk drives 440 are started up in group 470.

In FIG. 11, magnetic disk drives 551–560 are represented as circles.Since this example is aimed at achieving a very high transfer rate, anumber of parity groups each constituted of a plurality of disk drivesare arranged in parallel and reading and writing operations areconducted with respect to all of the disk drives simultaneously. Morespecifically, when data to be stored is received from the host 210, itpasses through an interface 230 to a first sequencer 510 where it issubdivided into units of an arbitrary size. These data units aretemporarily stored in first buffers 511–513 and then are furthersubdivided in sequencers 521–523. The subdivided data are written to themagnetic disks 551–560 via second buffers 531–540 and logical controlcircuits 541–550. This process is reversed during data regeneration.

Although it is possible to provide each disk drive in this arrangementwith its own separate power supply, the number of power suppliesrequired would be very large. A better arrangement can be realized bytaking advantage of the fact that each parity group (constituted by thedisk drives under one of the sequencers II) includes a redundant orparity disk drive. If power supplies are provided so that each suppliespower to only one disk drive in each parity group, specifically if apower supply 571 is provided to supply power to disk drives 551, 555,and 558, a power supply 572 is provided to supply power to disk drives552, 556 and 559 and so on, reading and writing will be possible withECC even if one of the power supplies should break down. It is thuspossible to realize a system with high reliability.

Being common to the whole system, the interface 230, the sequencer 510,the buffers 511–513 and the sequencers 521–523, for ensuring highreliability, preferably should be provided with a power supply separatefrom that for the magnetic disk drives to establish a dual power system.Even though the buffers 531–540 and control circuits 541–550 are part ofthe same power groups, they are logical circuits and preferably shouldhave a different voltage power supply. It is, however, contemplated tosupply them with stepped–down voltage from a power supply for drivingthe disk drives.

The power groups constituted such that each power supply supplies powerto one magnetic disk drive in each parity group in the manner of FIG. 11are started with exactly the same method and arrangement as in FIG. 1and the effect of the invention is thus manifested.

The power supplies of disk drives are sometimes equipped with batteriesfor supplying power during power outages and emergencies. When theinvention is applied to such a battery backed up system, it reduces theload on the batteries when they are used for starting up the disk systemand further upgrades system reliability.

In the foregoing, the invention was explained with respect to systemsemploying magnetic disk drives. It is obvious, however that theinvention can also be applied with good effect to systems employingoptical disk drives, hard or floppy disk drives, or the like insofar asthe spindle motors of the drives exhibit characteristics like thoseshown in FIG. 4.

Moreover, the description up to this point has been limited to apower-on sequence or start-up for the disk drive groups which enablespower to be supplied to the disk drives with high efficiency atstart-up. However, a disk drive puts an increased load on its powersupply from certain elements not only at power-on but also 1) duringseek operation when a disk actuator equipped with a plurality ofmagnetic heads operates to position the heads at target track positionson the disks, and 2) when a read-write amplifier is operated forconducting read and write operations. Those operations also involve arisk of power supply overload should they be conducted with respect toall the disk drives of a power group at the same time. When the presentinvention is applied, however, since the disk drives are organized ingroups and the operational timing is shifted between the respectivegroups, overloading of the power supplies can be avoided.

In accordance with this invention, since disk drive groups are startedup one at a time, the power supplies for powering the disk drives neednot be larger than necessary for supplying current required by all thedisk drives in steady state operation. Moreover, the time required forall of the disk drives to reach their rated rotational speed can beshortened.

Moreover, in a system which must as a whole be operated by battery, theapplication of the present invention, through its effect of reducing thetime required for start-up and its effect of suppressing the maximumload current, enables the use of small capacity batteries.

FIG. 12( a) is a block diagram of a part of the system relating to datastorage and FIGS. 12( b) and 12(c) are views for explaining the datastorage. As shown in FIG. 12( a), a CPU 1 is connected by a bus to anarray disk controller (ADC) 2 and an array disk unit (ADU) 3.

The ADU 3 has a plurality (six being specifically shown) of paritygroups 4, each of which has a group controller (GC) 5, four data diskdrives D (7) and one parity disk drive P (8). The system inputs andoutputs data between the data and parity disk drives 7, 8 and the CPU 1via data lines 6. Electric current is supplied from a power supply 10 tothe respective parity groups 4 via power lines 9. The number of datadisk drives 7 and parity disk drives 8 is determined in light of theamount of power the system is capable of supplying.

Each parity group 4 has a unit for generation of parity bits. One paritybit is generated from the data bits of the respective data disk drives7. Each of the data disk drives 7 and the parity disk drives 8 in theparity groups 4 is of the structure illustrated in FIG. 13.

The disks of this drive rotate at 3,600 rpm (requiring 16.6 ms perrevolution) and the data transfer rate from the disks is 3 MB/s. Data isrecorded on concentric tracks on the disks. The track positions aredefined on each disk by fixed positions of a single R/W head 13. R/Wheads #0 to #8 are positioned at corresponding track positions on thedisk 12 by an actuator 11. The actuator 11 moves all of the R/W heads 13simultaneously and by the same distance. A single positioning operationby the actuator 11 determines 9 tracks corresponding to the R/W heads #0to #8 and these 9 tracks are collectively referred to as a cylinder. Theamount of data that can be read from one disk during one revolution iscalled the track data capacity. Where this capacity is 35 KB and thereare 9 R/W heads per cylinder, the cylinder data capacity becomes35×9=315 KB.

Presuming a system of the type shown in FIG. 12( a), which has fiveparity groups 4, an example will now be explained regarding a case inwhich the CPU 1 issues a 10 MB write request and the system conductsparallel writing of this data. (In this and the following embodiments,data read can be considered to be conducted in the same manner as datawrite). It will be understood from FIG. 14 that under the conditionsjust defined 2000 KB of data will be written to each of the five paritygroups 4 to handle the 10 MB request. Thus it is necessary to write 500KB of data to each of the four data disk drives 7 in each parity group4.

FIG. 15 is a time chart relating to the processing conducted withrespect to the data disk drives 7 and the parity disk drives 8 of theparity groups 4 in this case.

Of the data transferred from the CPU 1 to a disk drive, data up to 315KB is stored in cylinder #1 and the remaining 185 KB is stored incylinder #2. The head selector 14 first selects R/W head #0 according toa first seek operation and this head writes data to the correspondingtrack starting from the position of the index. This index serves asreference for the start of data writing. When writing of an amount ofdata corresponding to one revolution of the track under R/W head #0 hasbeen completed, R/W head #1 is selected and an amount of datacorresponding to one track is similarly written starting from the index.The switching between heads is done electrically and the time requiredtherefor is substantially negligible. The aforesaid write processing iscontinued with R/W heads #2, #3, #4 . . . #8, after which the actuator11 moves the R/W heads #0 to #8 in unison to the next cylinder accordingto a second seek operation. From the foregoing it will be understoodthat the time required for writing data to one cylinder is 9 times thetime for one revolution of the disks 12, i.e. about 150 ms. When thewriting of data is continued to an adjacent cylinder in the aforesaidmanner, it is only necessary for the actuator to carry out a seekoperation for moving the group of R/W heads to the adjacent cylinder.This requires a seek time of about 3 ms, during which the disks rotateby 3/16.6 revolution or approximately ⅕ revolution. There is thus a waituntil the index next arrives and writing can begin. Including this waittime, therefore, there occurs a period of 16.6 ms (equal to the periodof one disk revolution) during which data transfer is impossible. Inother words, it is necessary to complete the seek operation within theperiod of one revolution. The amount of current consumed at this time isgenerally about 1.7 A per data disk drive 7 or parity disk drive 8. Whena seek operation is conducted, however, the amount of current requiredfor this operation is the total amount of current required up to amaximum of 4.5 A. Since each parity group 4 includes four data diskdrives 7 and one parity disk drive 8, i.e., a total of 5 disk drives,which perform the same operations, the maximum amount of currentrequired by the parity group 4 during seek operation becomes 4.5A×5=22.5 A. The power supply 10 supplies this current to the GCs 5 ofthe respective parity groups 4 via the power lines 9.

As shown in FIG. 16, each of the GCs 5 comprises a command processor 18,a disk control 19, a data processor 20 and a parity generator 21. Thecommand processor 18 processes commands between the ADC 2 and the group4. Based on instructions received from the command processor 18, thedisk control 19 carries out specific control within the parity group 4.The data processor 20 handles the subdivision and rebuilding of databetween the host 1 and the parity group 4. Associated with the dataprocessor 20 is a parity generator 21 for generating parity bits duringdata write and for reconstructing the data stored in a disk drive of theparity group 4 which has become unreadable because of a breakdown.

The disk control 19 synchronizes the rotation of the data disk drives 7and parity disk drive 8 of the parity group 4 and controls the timing ofthe parallel processing within the parity group 4 exactly as if it werebeing conducted with respect to a single disk drive. It also maintains acheck on whether the disk drives within the parity group 4 are operatingnormally and manages the supply of power within the parity group 4.

Timing control for rotation synchronization among the parity groups 4 isconducted by the ADC 2.

The ADC 2 subdivides the data transferred from the CPU 1 and allocatesthe subdivided data to the parity groups 4. It also controls therotation synchronization timing of the respective parity groups 4 forensuring that it does not become necessary to conduct a seek operationin any two of the parity groups 4 simultaneously. The specifics of thiscontrol are illustrated in FIGS. 12( b) and 12(c). FIG. 12( b) shows therelationship among the indices 32 on the disks among therotation-synchronized data and parity disk drives 7, 8 of the respectiveparity groups 4. During parallel processing the positions of the indicesfrom which data write is started are deliberately offset among thedifferent groups as shown in FIG. 12( b).

The data transferred from the CPU 1 is subdivided by the ADC 2 and thesubdivided data is simultaneously transferred to all of the paritygroups 4. Within the respective parity groups 4, the data received isonce stored in a buffer within the data processor 20 and is thenindependently stored in the data and parity disk drives 7, 8 of therespective groups. The data storage timing among the parity groups 4 atthis time is indicated in FIG. 12( c). The data storage time isindicated for five different parity groups, namely parity group #1through #5. With respect to each group, there is a data transfer time40, a seek time 41, and a second data transfer time 42. These times areshown in synchronism with respect to each other and in synchronism withthe seek current 43 according to the present invention when the indicesare offset, which seek current has a maximum value of 14 A, calculatedby multiplying the 2.8 A current required for each group, as explainedabove, times the five groups used in the example. In dotted lines, thereis shown the total seek current 44 that would be required if the indiceswere not offset, according to the prior art, which would require a seekcurrent of 70 A calculated by multiplying the same 2.8 A for one diskdrive times the five disk drives that are in use at the same time timesthe five groups. Therefore, the advantage of offsetting the indices isclearly shown.

As shown in FIG. 15, when the amount of data to be stored to a data diskdrive 7 in the parity groups #4 exceeds the capacity of one cylinder, itbecomes necessary to conduct a seek operation for switching to anothercylinder and, as a result, there occurs a one-revolution wait period inthe course of data storage during which processing cannot be conducted.Thus, as shown in FIG. 14, the rotation of the disk drives within therespective parity groups 4 is synchronized such that the index of eachparity group 4 making up the array of parity groups 4 is offset relativeto the indices of the other parity groups 4 by at least the seek time.As a result, the timing of the start of seek operation comes to beoffset among the parity groups 4. The amount of current that would haveto be supplied to the system should the seek operation start timing notbe offset would amount to 22.5 A×5=122.5 A, the seek current portion ofwhich is 2.8 A×5×5=70 A. In contrast, application of the inventionreduces this to 4.5 A×5=22.5 A, the seek current portion of which is 2.8A×5=14 A. The method in which this offset of the seek start timing isimplemented is decided in light of the amount of current the system iscapable of supplying. For example, where a current of 45 A is availablefor supply to five parity groups 4, it is possible to control therotation synchronization timing of the parity groups 4 as shown in FIG.17.

Since the invention enables operation at a lower seek current, therequired current capacities of the power supply equipment and theemergency backup battery are also smaller, according to the presentinvention, which are considerable advantages.

The array disk system of FIG. 18 is like that of FIGS. 12, 11 butinstead of offsetting the indices among the groups, the embodiment ofFIG. 18 rotation synchronizes the disk drives of the parity groups 4,making up the array of parity groups 4 conducting parallel processing,such that the indices thereof are in phase. While this makes it possibleto carry out data storage simultaneously with respect to all paritygroups 4 constituting the array of parity groups 4 in FIG. 18, withoutthe present invention it causes all of the seek operations to occur atone time. In this embodiment, therefore, the seek operation start timingsignals are offset among the five parity groups 4, #1, #2, #3, #4, & #5in FIG. 18, making up the array of parity groups, such that the seekoperations start at different timings from the indices for thecorresponding different groups.

A seek operation ordinarily begins as soon as data storage to thecylinder has been completed up to the track under R/W head #8. In FIG.18, however, the seek operation start timing is deliberately offset bydifferent amounts among the different parity groups 4, respectively. Themethod in which the seek operation start timing is offset among theparity groups 4 will now be explained.

As shown in FIG. 15, when an amount of data exceeding the capacity of asingle cylinder is to be stored in a data disk drive 7 of a parity group4, a seek operation becomes necessary for moving to the adjacentcylinder.

The GC 5 begins storing data to the track concerned after detecting theindex thereon. When a seek operation occurs in the course of storingdata, it takes about 3 ms to move the R/H heads to the neighboringtracks at which data storage is to be continued. By the time the seekoperation has been completed, the disk has rotated by 3/16.6 revolution.When this period is up, the index has already passed by the head so thatit is necessary to wait until the index comes around again.

Thus, even though the seek operation itself is completed in 3 ms, inactuality a wait period (a period during which processing cannot beconducted) equal in length to the time required for one revolution ofthe disk occurs in the course of data storage. Since the system simplywaits during this period and cannot carry out any processing, this timecan be used for sequentially completing the seek operations in therespective parity groups 4.

In each parity group 4, the seek operations are managed by the diskcontrol portion 19 in the GC 5.

The ADC 2 sets the seek offset times for the respective parity groups 4making up the array of parity groups 4 in advance and forwardsinstructions indicating these times to the disk control portions 19 ofthe GCs 5. Based on the instruction it receives, the disk controlportion 19 does not initiate a seek operation as soon as storage to thetrack under R/W head #8 has been completed but instead delays the startof the seek operation by the offset time indicated in the instructionfrom the ADC 2. Taking the specific example of conducting parallelprocessing with the five parity groups 4 shown in FIG. 18, while a seekoperation is started immediately after completion of storage to thetrack under R/W head #8 in the disk drives of group #1, the initiationof the seek operation in group #2 is delayed until the seek operation ingroup #1 has been completed. The seek operations in the remaining paritygroups 4 are similarly offset such that seek in group #3 is startedafter completion of seek in group #2, seek in group #4 after completionof seek in group #3, seek in group #5 after completion of seek in group#4, and so on. The data and parity disk drives 7, 8 of the respectiveparity groups 4 are rotation synchronized and seek operation occurssimultaneously in all the data and parity disk drives 7, 8 of the sameparity group 4. The time required for the seek operation to move the R/Wheads to the adjacent track is about 3 ms. Thus where the seek starttimes are offset in the foregoing manner, the seek operations in all ofthe parity groups 4 involved in the parallel processing are completedwithin the one-revolution wait period during which data transferprocessing is impossible.

The seek operation timing is automatically controlled by controlling theseek operation of the GCs 5. In this method, even when it becomesnecessary to write to the next cylinder because the amount of dataexceeds the capacity of a single cylinder, the CPU 1 is not made awareof this fact and the seek operation for moving to the adjacent track forcontinuing the writing of data is conducted automatically by the GC 5.

The offsetting of the seek operations can be realized by the simpleexpedient of using software techniques to offset the times at which theADC 2 issues its commands. Moreover, since the disk indices are all inalignment, control for rotation synchronization is easy to conduct.

Alternatively, it is possible to offset the locations at whichinformation indicating seek operation start time are recorded on thedisks.

With the operating systems (OS's) currently used in mainframe computers,it is not possible with a single input/output request to process a largeamount of data bridging a plurality of cylinders as is the case in seekEmbodiments 1 and 2. The maximum amount of data that can be processed bya single input/output request is limited to the capacity of a singlecylinder. Thus where a large amount of data bridging a plurality ofcylinders is processed, it is necessary to allot one input/outputrequest per cylinder and the host is required to issue as manyinput/output requests as there are cylinders involved. When, forexample, parallel processing is conducted in the manner shown in FIG.18, namely where data is stored bridging cylinders #1 and #2 in fiveparity groups 4, the CPU 1 of the mainframe issues a single input/outputrequest for storage of data to cylinder #1 and then, after seekoperation has been completed following issuance of a seek command,issues another single input/output command for storage of data tocylinder #2.

In each instance, the CPU 1 issues only one seek command for all of theparity groups 4 making up the array conducting parallel processing. TheADC 2 issues this seek command to the GCs 5 of the respective paritygroups 4 conducting the parallel processing but, as shown in FIG. 18(b), in doing so it offsets the issuance of the seek command among theGCs 5 of the different parity groups 4, thus offsetting the seekoperation start timing.

When this method is used, it becomes possible, similarly to the case ofthe other embodiments, to complete seek operation with respect to all ofthe parity groups 4 involved in the parallel processing within theone-revolution wait period during which data transfer processing isimpossible.

Data is stored at the same head address and the same cylinder address inall of the parity groups 4 making up the array of parity groups 4conducting parallel processing. As shown in FIG. 19, in the presentembodiment the head address from which data storage is started is variedamong the groups at the time of starting data storage. While the ADC 2sets the same data storage start address for all parity groups 4 makingup the array of parity groups 4 in advance and forwards an instructionindicating this time to the disk control portions 19 of the GCs 5, thecommand processing portions 18 of the GCs 5 changes this head address.

Consider, for example, the arrangement shown in FIG. 19( a) in whicheach of the data disk drives 7 and parity disk drives 8 of the fiveparity groups 4 has five disks and read/write is conducted with respectto only the upper surface of each disk. In this case, each cylinderconsists of five tracks. Where data is an amount equal to the capacityof six tracks is to be stored in each group, in group #1 writing of datais started from head #1, proceeds through heads #2, #3, #4 and #5, andthen, following a seek operation, continues at head #1 of the nextcylinder. In group #2, storage starts from head #2, proceeds throughheads #3, #4 and #5, and then, after a seek operation, continues atheads #1 and #2 of the next cylinder. In group #3, storage starts fromhead #3, proceeds through heads #4 and #5, and then, following a seekoperation, continues at heads #1, #2 and #3 of the next cylinder. Ingroup #4, storage starts from head #4, proceeds through head #5, andthen, following a seek operation, continues at heads #1, #2, #3 and #4of the next cylinder. In group #5, storage starts from head #5 and then,following a seek operation, continues at heads #1, #2, #3, #4 and #5, ofthe next cylinder. Varying the head address for the start of datastorage among the different parity groups 4 in this manner also causesthe seek operation start timing to be offset among the different paritygroups.

The ADC 2 sends the same data storage start address instruction to thedisk control portions 19 of the GCs 5 of all the parity groups 4 makingup the array and the individual command processing portions 18 changethis address. It is obvious, however, that the same effect can also beobtained by having the ADC 2 change the data storage start address andsend instructions based on the changed address to the GCs 5.

In the above description, it was explained how, by offsetting the seekoperation start timing among different parity groups 4, the inventionachieves its object of preventing the occurrence of the large currentwhich arises when a large number of seek operations occur simultaneouslyin an array of parity groups 4 conducting parallel processing. This samethinking can obviously also be applied for offsetting read and writestart timing so as to reduce the amount of current required during theseoperations.

The head addresses can be varied by software techniques, which isadvantageous in that it expedites the control for seek operation offset.

While the foregoing relates to cases in which the seek start timing isoffset among the groups, the same results can be obtained by offsettingthe seek operation start timing among the disk drives in one and thesame group.

Application of the invention reduces the current to be supplied to thearray disk system and also reduces the capacity required of a batteryprovided for preventing loss of data owing to power outages and thelike. It thus becomes possible to provide battery backup over a longperiod of time with a battery of relatively small capacity, thusenhancing reliability against the risk of data loss resulting from poweroutages. Furthermore, the invention also makes it possible to reduce thesize of the system's power supply equipment.

With respect to FIG. 19( b), each of the first and second data transferperiods 40, 42 respectively for the groups #1 to #5, is divided byvertical dashed lines. For group #1, data transfer period 40 is dividedinto subperiods for head #1, head #2, head #3, head #4, head #5respectively proceeding from left to right for a first cylinder and thesecond data transfer period 42 is for head #1 of cylinder 2. Withrespect to the second parity group #2, the data transfer period 40 isdivided into 4 subperiods respectively for head #1, head #2, head #3,head #4 of cylinder #1, and the second data transfer period 42 isdivided into two subperiods for head #1 and head #2 of cylinder #2. Withrespect to parity group #3, #4, #5, the data transfer periods 40 arerespectively divided into 3, 2 and 1 subperiods for head numbers 1, 2, 3head numbers 1, 2 and head numbers 1, respectively, each for cylinder 1;the second data transfer period 42 is divided respectively into 3, 4 and5 subperiods respectively for head numbers 1, 2, 3 head numbers 1, 2, 3,4 head numbers 1, 2, 3, 4, 5 each for cylinder #2 of the respectiveparity groups. The seek current 41 for each group is shown. With this,it is seen that the current 43 has five current surges at timings 41, asshown, each of which is 14 amps, obtained by multiplying the 2.8 amps by5. It is noted that according to the present invention the currentsurges do not overlap and therefore do not reinforce each other.

While a preferred embodiment has been set forth along with modificationsand variations to show specific advantageous details of the presentinvention, further embodiments, modifications and variations arecontemplated within the broader aspects of the present invention, all asset forth by the spirit and scope of the following claims.

1. A method of starting up disk drive spindle motors in an array disksystem having disk drives organized into groups which are started upseparately so as to reduce the amount of electric current required bythe array disk system, said method comprising: supplying current from apower supply to start up a first group of said spindle motors initially,said first group of said spindle motors started up initially being morethan one spindle motor and less than all of said spindle motors; andthen supplying current, from the same power supply that supplied currentto said first group of said spindle motors initially, to additionallystart up one or more of said spindle motors other than said first groupof said spindle motors started up initially; wherein the current that issupplied to start up said first group of said spindle motors initially,and the current that is supplied to additionally start up said one ormore of said spindle motors, are each less than the current that wouldbe required to start up all of the spindle motors in the array disksystem at the same time; and wherein a sum of a time that is requiredfor said first group of spindle motors to reach steady-state fromstart-up by supplying said current initially, and a time that isrequired for said one or more of said spindle motors additionallystarted up to reach steady state from start-up by supplying current fromsaid power supply, is less than a time that would be required to startup all of said spindle motors in the array disk system one-by-one.
 2. Amethod of starting up disk drive spindle motors in an array disk systemas claimed in claim 1, wherein said supplying steps are performed suchthat a spindle motor in a start-up is supplied with a start-up current,and a spindle motor at steady-state is supplied with a steady-statecurrent that is lower than said start-up current.
 3. A method ofstarting up disk drive spindle motors in an array disk system as claimedin claim 1, wherein the time between power switch-on of the overallarray disk system and start of driving the spindle motors is setindependently for each of the groups of the disk drives so as to preventoverlap of the initial currents among the groups.
 4. A method ofstarting up disk drive spindle motors in an array disk system as claimedin claim 1, wherein the number of the disk drives constituting theindividual groups decreases in the order that the groups are started up.5. A method of starting up disk drive spindle motors in an array disksystem as claimed in claim 4, wherein after the start-up of the firstgroup of spindle motors, the reserve power of a power supply thatsupplies the current to the first group of spindle motors is equal tothe rated capacity of the power supply minus the amount of currentrequired for maintaining the disk drives of the first group in thesteady state.
 6. An array disk system having disk drives organized intogroups which are started up separately so as to reduce the amount ofelectric current required thereby, said system comprising: a pluralityof disk drive spindle motors; and a power supply electrically connectedto said plurality of disk drive spindle motors, wherein said powersupply supplies current to start up a first group of said spindle motorsinitially, said first group of said spindle motors started up initiallybeing more than one spindle motor and less than all of said spindlemotors, and then additionally supplies current to start up one or moreof said spindle motors other than said first group of said spindlemotors started up initially; wherein the current that is supplied tostart up said first group of said spindle motors initially, and thecurrent that is supplied to additionally start up said one or more ofsaid spindle motors, are each less than the current that would berequired to start up all of the spindle motors in the array disk systemat the same time; and wherein a sum of a time that is required for saidfirst group of spindle motors to reach steady-state from start-un bysupplying said current initially, and a time that is required for saidone or more of said spindle motors additionally started up to reachsteady state from start-up by supplying current from said power supply,is less than a time that would be required to start up all of saidspindle motors in the array disk system one-by-one.
 7. An array disksystem as claimed in claim 6, wherein said power supply supplies aspindle motor in a start-up with a start-up current, and supplies aspindle motor at steady-state with a steady-state current that is lowerthan said start-up current.
 8. An array disk system as claimed in claim6, wherein the time between power switch-on of the overall array disksystem and start of driving the spindle motors is set independently foreach of the groups of the disk drives so as to prevent overlap of theinitial currents among the groups.
 9. An array disk system as claimed inclaim 6, wherein the number of the disk drives constituting theindividual groups decreases in the order that the groups are started up.10. An array disk system as claimed in claim 9, wherein the reservepower of the power supply after the start-up of the first group is equalto the rated capacity of the power supply minus the amount of currentrequired for maintaining the disk drives of the first group in thesteady state.